I was looking for the same thing -- I have a list of users and would like to filter it so it only shows members of a certain group, and I need the filter to be a dropdown select list, not a field where you have to type in the group ID.

Then in Views -
1.) Add a relationship for OG membership: OG membership from Node
2.) Add a relationship for OG membership: Group Node from OG membership that uses the relationship OG membership from Node.
3.) Add the filter criteria Field: Groups audience (og_group_ref) and expose it to visitors
4.) Configure the filter to your preference.

(Additionally, I use Better Exposed Filters to make my drop down filters in Views a little more aesthetically pleasing.)

I'm still trying to get my mind around how OG 2.x and Views interact. I wish that I could provide a more elaborate response, but hope this is somewhat helpful.

This is old but still applies to newer versions of OG and Drupal. I followed the directions in #3 and was able to get this working, but only partially. This does not work as expected if you are using Groups and Sub Groups like I am.

I am creating a calendar for a district website that acts like a multisite with OG. The District is the top level group, with each School in the district as subgroups.

This filter only looks at the top level group, so if I toggle District, the events show, if I only toggle a School, no events show.